1. Further simplify location code so that it's even easier to set up and maintain large location hierarchies.
2. Get everything working in one city.
3. Get everything working in two cities.
4. Handle travel between cities and prevent casual cross-country raids.
5. Generalize the "needs car" thing so that what areas you can reach without a car depends on where your squad is based.
6. Ensure the display tags on cities are useful. I mean the <safe house> tags.
7. Have cities display with a short description on the right, especially for the benefit of players not familiar with US cities.
8. Any left over testing and quality control.
I thought setting up the cities I'd want to test with would be easy, but it's actually been quite a design puzzle to give multiple parallel cities a sense of place and accuracy to the locations, while also mixing in playability.
I'm going to start out with three cities: New York, Los Angeles, and Seattle. New York and LA because they're the two biggest cities in the country, they're geographically on opposite corners, they're very well known, and they're two of the three major US cities that have been modeled in Grand Theft Auto. Seattle is included as the starting city, or at least the default starting city -- it is the city LCS's layout and district names are based on, so it has almost no changes. At first I wanted to make everything follow a similar template to ensure that players wouldn't have to re-learn the city layouts when visiting new cities, but testing quickly showed that this made the cities feel like they weren't different cities, which kind of goes against the point. My best judgment now says that not knowing where a given site is in a new city, at least at first, is a small sacrifice for rule of cool; the districts will have unique names for each city, so it's not like it'll be all that surprising that they have unique contents anyway.
While I've decided to mix things up, I'm still giving each city roughly the same categories of locations: A downtown area, a less dense middle or high end area, a seedier low end area, and an outlying district. The dominant economic activities of these areas can vary based on what the nature of the given city is, however, so their contents can vary significantly. You might find that New York City has its army base in "Brooklyn and Queens" while its prison is on "Manhattan Island". Meanwhile, Los Angeles has its club, CEO House, and most expensive apartments in the "Greater Hollywood" district rather than downtown or in the outskirts, while its "Seaport Area" gets both the research labs and the industry.
I'm taking a relatively light touch with limiting distribution of the current locations: Cable News is only found in New York, CEO House is unique to Los Angeles, and Intelligence HQ is unique to Seattle. All three locations have prisons and army bases; I couldn't realistically give Seattle a nuclear plant, since the nearest one is across the State, but the other two have them. I gave each location one of the three CCS safehouses as a temporary measure; the CCS wasn't designed for multi-city play, but I can only fix one mechanic at a time for it, so they'll be on the backburner for a bit.
Ultimately, all this is a first draft. Like army battle plans, it may not survive first brush with live tests. But it's a suitable groundwork that I can use to start testing mechanics with. I also want to give fair warning that simply having cities won't make the game better -- it's everything we can do with the game after we have cities that makes this line of development exciting. The game won't play very differently until and unless some of the neat things that have been suggested in this thread are implemented: local newspapers, police hunt differently by city, cities with different political biases, CCS cells in different cities, ability to go to DC and visit unique locations there... the game has to capitalize on the ability to visit different cities before it really makes a difference.